a) Field of the Invention
The present invention relates to a group III-V compound semiconductor device, and more particularly to a group III-V compound semiconductor device of a high electron mobility transistor (HEMT) type.
b) Description of the Related Art
A conventional high electron mobility transistor (HEMT) uses mainly i-type GaAs as an electron transfer or travelling layer epitaxially formed on a GaAs substrate, and uses n-type AlGaAs as an electron supply layer epitaxially formed on the electron transfer layer. The term "electron transfer layer" used herein means a layer in which electrons transfer, and the term "electron supply layer" means a layer from which electrons are supplied to the electron transfer layer. Si is generally doped heavily in the electron supply layer as n-type impurities to supply the electron transfer layer with electrons (carriers).
An n-type GaAs cap layer is formed on the electron supply layer. Also formed on the cap layer are source and drain electrodes in ohmic contact with the cap layer and a gate electrode in Schottky contact with the cap layer. A spacer layer is sometimes inserted between the electron transfer layer and electron supply layer. The spacer layer has the same composition as the electron supply layer except that it does not contain impurities.
A deep energy level called a DX center is formed in an Si-doped AlGaAs. The device properties of an AlGaAs/GaAs based HEMT are limited by the DX center. The DX center greatly degrades the device properties when the device is operated at a low temperature such as at the temperature of liquid nitrogen.
From this reason, a HEMT with an electron supply layer not containing the DX center has drawn attention. As such an electron supply layer not containing the DX center, InGaP or InAlAs based material has been considered.
For the mass production of InGaP based HEMTs, it is preferable to use metal organic vapor phase epitaxy (MOVPE) which can use P which has a high vapor pressure and is flammable. For the uniform growth, reduced pressure MOVPE in a depressurized furnace is required to be performed.
Phosphine (PH.sub.3) generally used as a source of P has a high decomposition temperature. Therefore, in a depressurized furnace having a high flow velocity, most of phosphine passes over a substrate before it is decomposed sufficiently.
In order for P to be supplied sufficiently, it is necessary to supply a great amount of phosphine. Supplying a sufficiently large amount of phosphine as the P source allows a high quality InGaP layer to be grown.
However, the device properties of an InGaP based HEMT actually manufactured are not so good as expected. Such a difficult issue was reported by many researchers. The phenomenon that the device properties of a HEMT using an Si-doped InGaP electron supply layer are inferior to those of a HEMT using an undoped InGaP electron supply layer may be reasoned at least partially from the presence of Si.
In order to suppress the effects of solid phase diffusion of Si, it is known to insert an undoped InGaP spacer layer between an Si-doped InGaP electron supply layer and i-type GaAs electron transfer layer. Also in this case, the device properties are degraded if the thickness of the spacer layer is as thin as 50 angstroms or less.
If the doping amount of Si is set to 1*10.sup.17 cm.sup.-3, a good mobility of about 23000 V.sup.2 cm.sup.-1 s.sup.-1 can be obtained at 77K, but the two dimensional electron gas concentration becomes as low as 5*10.sup.11 cm.sup.-2. The low concentration of two dimensional electron gas lowers a transfer conductance and the like of a HEMT device to the extent that the device becomes improper in practical use. The two dimensional electron gas concentration is preferably 1 to 2*10.sup.12 cm.sup.-2 or higher.
If the doping amount of Si is increased to 1.4*10.sup.18 cm.sup.-3, a sufficient two dimensional electron gas concentration of 1.8*10.sup.12 cm.sup.-3 can be obtained at 77K, but the mobility lowers to 1400 V.sup.2 cm.sup.-1 s.sup.-1.
As above, one of the two dimensional electron gas concentration and mobility can be improved by adjusting the doping level of Si, but the other factor is degraded to the degree of inability of practical use.
It has been proposed to use an undoped AlGaAs as the spacer layer for the prevention of Si diffusion from an Si-doped inGaP electron supply layer. However, if Si is diffused in AlGaAs, the DX center is generated resulting in a hardship of operation at a low temperature such as at the temperature of liquid nitrogen.
The present main trend is to reduce the thickness of the non-doped spacer layer as thin as possible, in a typical case to zero, because this structure provides a high concentration of two dimensional electron gas, making an undoped AlGaAs spacer layer out-of-date. An interface control becomes therefore very important in order to realize a good device property without using the non-doped spacer layer.
A technology of manufacturing a novel HEMT not using an Si-doped AlGaAs electron supply layer has not been developed to a sufficient level up to date.